1. Field of the Invention
The present invention relates to a boosting circuit for boosting an input voltage and obtaining an output voltage, and in particular, relates to a technique which improves the efficiency of a charge pump boosting circuit.
2. Description of the Related Art
In a charge pump boosting circuit, generally, a charging cycle which carries out charging with respect to a boosting capacitor from the input side, and a discharging cycle which transfers the accumulated charges of the boosting capacitor to the output side, are carried out repeatedly. In the charge pump boosting circuits disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2002-369500 and Reissue Publication of International Patent Disclosure WO 2002/061931, a differential amplifier, at which input voltage is applied to the non-inverting input terminal, and a switching element, which turns on in the charging cycle, are provided at the input side, and a switching element, which turns on in the discharging cycle, is provided at the output side. A portion of the output voltage is fed-back to the inverting input terminal of the differential amplifier.
By connecting the above-described, conventional, charge pump boosting circuit in series, an output voltage which is several times the input voltage can be obtained. Here, in a case in which a high output voltage, e.g., output voltage which is N times the input voltage, is obtained at a conventional boosting circuit, the load current which flows at the output side also becomes N times. Therefore, current of a magnitude of the same extent as this load current must be made to flow into the boosting capacitor.
However, in the conventional charge pump boosting circuit, when an attempt is made to obtain a high output voltage, it is difficult to make current of the same extent as the load current flow-into the boosting capacitor due to the on resistances of the transistor provided within the differential amplifier and the transistor which structures the switching element. Namely, in the conventional charge pump boosting circuit, the higher the output voltage that is attempted to be obtained, the more the boosting efficiency worsens.
On the other hand, in order to decrease the on resistance of the transistor, there exists a method of expanding the gate width, and the like. However, this method is not preferable because it leads to an increase in the surface area occupied by the devices for mounting the boosting circuit.
From the above standpoints, a charge pump boosting circuit, which improves the current driving ability with respect to the boosting capacitor without causing an increase in the surface area occupied by devices, is desired.